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GPDs studies at COMPASS

GPDs studies at COMPASS. Etienne Burtin, CEA/Saclay, DAPNIA/SPhN on behalf of the COMPASS collaboration. DVCS studies for a GPD experiment in 2010 Physics impact Experimental issues Recoil detector prototype Exclusive r 0 production in present COMPASS data Principle

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GPDs studies at COMPASS

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  1. GPDs studies at COMPASS Etienne Burtin, CEA/Saclay, DAPNIA/SPhN on behalf of the COMPASS collaboration DVCS studies for a GPD experiment in 2010 Physics impact Experimental issues Recoil detector prototype Exclusive r0 production in present COMPASS data Principle Preliminary 2002 results SIR workshop 2005 May 19, 2005 – Newport News, VA

  2. Generalized Parton Distributions g* g,p,r Factorisation: Q2 large, -t<1 GeV2 hard x+x x-x soft GPDs P P’ t Generalized Parton Distributions H(x,0,0) = q(x) measured in DIS for quarks : 4 functions H(x,x,t) F(t) measured inelastic scattering

  3. Single Spin Asymmetry Beam Charge Asymmetry Polarised beam +/- chargedbeam DVCS observables Deep VCS Bethe-Heitler High energy beam Lower energy=> use interference - holography Cross section COMPASS muon beam can do all !

  4. Collimators 1 2 3 4 H V H V scrapers T6 primary Be target Compass target Be absorbers Protons 400 GeV Muon section 400m Hadron decay section 600m Polarized μ+andμ-beams • Solution proposed by Lau Gatignon: • To select Pπ=110GeV andPμ=100GeV • to maximise the muon flux • 2) To keep constant the collimator • settings which define • the π and μ momentum spreads • Pol μ+ = -0.8andPol μ- = +0.8 • I(μ+) ~ 2. I(μ-) • switch in ~10 mins (once per shift?) Requirements for DVCS: -same energy -maximum intensity -opposite polarisation to a few % -change of beam charge frequent 2.108 muons/spill 1.3 1013protons/spill

  5. dσ(μpμp) = dσBH + dσDVCSunpol + PμdσDVCSpol + eμ aBHReADVCS + eμ PμaBHImADVCS μ’ * θ μ p φ  cos nφ sin nφ Extraction of GPDs in the case of μ+ / μ- Extraction of GPDs in the case of t, ξ~xBj/2 fixed Pμ+=-0.8 Pμ-=+0.8

  6. E=190, 100GeV Nx2 new LINAC 4 (SPS injection) in 2010 could COMPASS benefit from this ? Kinematical domain Collider : H1 & ZEUS0.0001<x<0.01 Fixed target : JLAB 6-11GeV SSA,BCA? HERMES 27 GeV SSA,BCA COMPASScould provide data on : Cross section (190 GeV) BCA (100 GeV) Wide Q2 and xbj ranges Limitation due to luminosity

  7. model 1 model 2 BCA  Sensitivity of BCA to models Model 1: H(x,ξ,t) ~ q(x) F(t) Model 2: from Goeke, Polyakov and Vanderhaeghen H(x,0,t) = q(x) e t <b2> = q(x) / xα’t COMPASS sensitivity to the different spatial distribution of partons  when xBj  Good sensitivity to models in COMPASS xBj range

  8. Projected errors of a possible DVCS experiment Beam Charge Asymmetry L = 1.3 1032 cm-2 s-1 Ebeam = 100 GeV 6 month data taking 25 % global efficiency 6/18 (x,Q²) data samples 3 bins in xBj= 0.05, 0.1, 0.2 6 bins in Q2 from 2 to 7 GeV2 Model 1 : H(x,ξ,t) ~ q(x) F(t) Model 2 : H(x,0,t) = q(x) / xα’t Good constrain for models

  9. Additionnal equipment to the COMPASS setup all COMPASS trackers: SciFi, Si, μΩ, Gem, DC, Straw, MWPC μ’ 2.5 m Liquid H2 target to be designed and built ECAL 1 or 2   12°  COMPASS equipment with additional calorimetry at large angle p’ μ Recoil detector to insure exclusivity to be designed and built

  10. Physical Background to DVCS Source :Pythia 6.1generated DIS events Apply DVCS-like cuts one m’,g,p in DVCS range no other charged & neutral in active volumes detector requirements: 24° coverage for neutral 50 MeV calorimeter threshold 40° for charged particles in this case DVCS is dominant

  11. 24° ECAL0 12° 4m Possible solution for the recoil detector proposed in the Workshop on the Future Physics at COMPASS 26 Sept2002 Received funding by EU FP6 (Bonn-Mainz-Saclay-Warsaw ) Goal : full test of a prototype of a 45 degree sector - scintillating material studies (timing at 200 ps over 4 m) - fast triggering and multi-hit ADC/TDC system Also studying a scintillating fibers solution (Monte-Carlo) with good timing provided by incoming or scattered muons

  12. Hard Exclusive Meson Production It comes for free with the recoil detector and existing COMPASS trackers Cross section: Vector meson production (ρ,ω,…) H & E Pseudo-scalar production (π,η…) H & E ~ ~ Hρ0 = 1/2 (2/3Hu + 1/3Hd + 3/8Hg) Hω= 1/2 (2/3Hu – 1/3Hd + 1/8Hg) H = -1/3Hs - 1/8Hg Can be investigated with present COMPASS data Single spin asymmetry ~ E/H for a transverse polarized target

  13. Compass Set-up 2002-2003 at CERN 250 physicists 26 institutes magnets muon filter Calorimeters 160 GeV pol. m beam ~ 200 detection planes Silicon, SciFi, Micromegas, Drift chambers, GEM, Straw chambers, MWPC RICH polarized target

  14. m’ p- m Mpp Assuming both hadrons are p: 0.5 < Mpp< 1 GeV r0 g* p+ N N’ Emiss Exclusivity of the reaction Emiss=(M²X-M²N)/2MN -2.5 < Emiss < 2.5 GeV Incoherent production: 0.15 < pt²< 0.5 GeV² scattering off a quasi-free nucleon pt² Incoherent exclusive r0 sample selection Event topology: • Kinematics : • n > 30 GeV • Em’ > 20 GeV • Q² > 0.01 GeV²

  15. r0 angular distributions f Definitions : in g*-p CM frame p’ m’ p g* m r° p+ F inr° rest frame q p- The angular distribution W(cosq,f,F) depends on the r0 spin density matrix elements  23 (15) measurable with polarised (unpol.) beam This analysis : one-dimension distributions  test of s-Channel Helicity Conservation SCHC Distributions are corrected for acceptance. Background not subtracted. Statistical error limited by MC sample (except last Q² bin).

  16. Measurement of r 04 00 Distribution : Spin density matrix elements: Tlg are helicity amplitudes meson photon

  17. Measurement of r and Im r 04 1-1 3 1-1 beam polarisation weak violation if SCHC holds: R can be extracted

  18. 2002 2003+ Determination of Rr° =sL/sT as SCHC holds …: only T00≠0 T11≠0 Then : Impact on GPD : sL is dominant at high Q² (factorisation only valid for sL) High statistics from g-production to hard regime better coverage at high Q² more data to come …

  19. Conclusions Exclusive vector meson production in COMPASS data - preliminary results from 2002 data - allow test of SCHC - shows dominance of sL at Q²>2GeV² - more data on tape with better coverage at high Q² to come - insight on GPDs : transverse asymmetry and cross section measurements Towards a GPD experiment using COMPASS… - COMPASS is complementary to other experiments - has good sensitivity to GPD models through BCA - has good Q² range for 0.03<xbj<0.2 - will be able to measure Hard Exclusive Meson Production - seeking collaborators… This initiative is now an “Express of Interest” : SPSC-EOI-005 http://doc.cern.ch//archive/electronic/cern/preprints/spsc/public/spsc-2005-007.pdf COMPASS should also be a good place to study GPD in 2010 !

  20. Gatignon compass Proton luminosity upgrade at CERN

  21. μ’ * θ μ p φ μ μ DVCS+ Bethe Heitler p p BH calculable The high energy muon beam at COMPASS allows to play with the relative contributions DVCS-BH which depend on 1/y = 2 mp Eℓ xBj/Q2 Higher energy: DVCS>>BH  DVCS Cross section • Smaller energy: DVCS~BH • Interference term will provide the DVCS amplitude

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